“We submitted a freedom of information request to NWAS, which made it clear the Trust that had sought advice from senior figures within NHS employers - a clear breach of national job evaluation guidelines (JEG).

“Our members now have no confidence in the JEG to enable a a fair and transparent evaluation.

“GMB is asking for an independent investigation to be undertaken into what has gone on during the past 13 years and the reason we are in this situation.

“Our members have been astoundingly patient in waiting for their job to be re-evaluated for the years 2005 to 2016. We are not taking this action lightly and would have expected the employer to take it more seriously.”

Michael Forrest, Deputy Chief Executive for North West Ambulance Service, said: “The trust is extremely disappointed with the actions proposed by GMB and believes it has worked closely with them to try and work through the issues which formed the basis of the ballot. Talks are ongoing and we will be doing all we can to minimise any impact.”